223 Howe and Underwood: The Genus Riella 



illumination, though no exact investigations were undertaken to 

 demonstrate the influence of light in this matter. The prevailing 

 form, however, while still unistratose, is spatulate, though flabel- 

 liform and linear outlines are not uncommon and grotesquely 

 lobed and branched stages are often met with. Linear or ribbon- 

 shaped forms, like that shown in /. j6, are especially common 

 when the young plants are from the first well covered with water. 

 Just below the chief growing point c, in the stage illustrated by 

 /. J4, there are cell divisions in the plane of the paper, which 

 possibly foreshadow the axis or stem, though they may have 

 arisen in this case through changed relations to the light. We 

 have not yet been able to follow the development much beyond 

 this point, but we have grounds for believing that the subsequent 

 history is essentially as described by Goebel (/. c.) for young plants 

 of A. Battandicri with the exception that the wing is probably 

 more lobed than in that species. In most cases, the development 

 is confined to practically a single plane, though occasionally, as in 

 the case represented by /. jj, the wing-lobes show an irregu- 

 larly spiral arrangement, due perhaps to changed positions in re- 

 gard to the light. 



Kxplaualiou of Plates 



[The drawings have been prepared by M. A. Howe, chiefly with the aid of a 

 camera lucida.] 



Plate ii. Riella Americana Howe & Underw. 



1 and 4. $ gametophytes, natural size. 



2 and 3. 9 gametophytes, natural size. 



5. Terminal portion of a branch of the $ gametophyte, with a young branch at 



the apex, X H- 



6. Terminal portion of a branch of the 9 gametophyte, X l*- 



7. An involucre, with mature sporogonium, X ^6. 



8. A scale, X 55- 



9. Portion of axis near the apex, showing gemmae and scales, X 40- The 

 gemmae are shown at a and below ; the scales are, in this case, nearer the wing. 



10-18. Stages in the development of the gemmae. See text, pp. 219 and 220. 



10. A trichome destined to become a gemma, in distal (outer) aspect, X 193- 



11. A similar stage in lateral view, X I93- 



12-15. Later stages ; 12, 13, and 15 in distal aspect, 14 in proximal, X ^93 5 ^^ 

 point of attachment. 



16. A more advanced stage, inverted as regards its original relations to the axis 



and as regards the preceding figures, X 193- 



17. Outline of a gemma in a later stage, X 55- • 



18. A young gametophyte derived from a gemma, X 28. 



